Semiconductor optoelectronic devices typically convert electrical energy into optical energy by taking advantage of the interaction of electrical energy with the semiconductor's crystal structure, which has a specific electronic energy configuration known as the electronic band structure. Semiconductor light emitting diodes (LEDs) generate light using semiconductor junctions comprising at least a p-type semiconductor region and an n-type semiconductor region. The p-type semiconductor region is designed to be a source of holes, whereas the n-type region is a source of electrons. Under the appropriate external electrical bias, electrons and holes are injected from their respective sources towards an intrinsic layer, which serves as an electron-hole-recombination (EHR) region. Group III-nitride (III-N) material is generally the most mature wide bandgap semiconductor material and is widely used in ultraviolet (UV) and visible LEDs in the wavelength range of 250 to 600 nm.
In semiconductor devices, the development of high crystalline structural perfection is necessary for achieving high performance in both electronic and optoelectronic devices. Group III-N epigrowth is typically performed on sapphire, silicon (Si) or silicon carbide (SiC) substrates, all of which have high lattice mismatch to III-N materials such as aluminum nitride (AlN) and aluminum-gallium-nitride (AlGaN). Growth of device stack epilayers on a dissimilar substrate material generates a large number of threading dislocations (e.g., on the order of 1010 cm−2) in the epistack. Threading dislocations are defects which propagate vertically through an epifilm, usually originating at the interface between the substrate and epifilm. Threading dislocation density in the intrinsic layer of a semiconductor LED device is an important factor in determining the internal quantum efficiency (IQE) and therefore light output intensity of LEDs, as they provide non-radiative recombination sites; that is, recombination without producing photons. The presence of defects also affects other operational parameters, such as leakage currents and lifetime of the device.
Some optoelectronic devices emit light in the deep ultraviolet (DUV) wavelength range (λ≤280 nm) using group III metal nitride semiconductor materials, such as aluminum gallium nitride (AlGaN). However, the optical emission intensity from such LEDs to date has been relatively poor compared to visible wavelength LEDs. It has been widely believed that a poor deep ultraviolet emission intensity in DUV LEDs is due to an inferior crystalline structural quality of deposited group III metal nitride materials which leads to poor electrical behavior of the LEDs. In comparison with other technologically mature group III-V compound semiconductors, such as gallium aluminum arsenide (GaAlAs), the group III metal nitrides exhibit crystalline defects at least two to three orders of magnitude higher. These defects reduce efficiency by causing radiationless EHR. The structural quality of the group III metal nitrides can be improved by epitaxial deposition on native substrates, such as, aluminum nitride (AlN) and gallium nitride (GaN).
In recent studies, the use of nanofibers in light emitting devices has been investigated since in general, thin fibers are less likely to have defects due to the relative relaxation in the x-y plane. For example, GaN and AlN nanofibers have been demonstrated to be virtually defect-free. Surface defect states that plague the nanofiber devices so far can be reduced or eliminated by a suitable growth and/or a cladding such as AlN or atomic layer deposition oxides as has been demonstrated in recent literature.
Mechanoluminescent (ML) materials are also being researched as a way for generating light. ML materials are typically solid materials, such as particles in a powder form, that emit visible light when mechanical stress is applied. Types of mechanical stress include deformation, friction, and impact.